Polyactic acid polymer composition for thermoforming, polylactic acid polymer sheet for thermoforming, and thermoformed object obtained therefrom

ABSTRACT

In order to provide a polylactic acid polymer composition to be thermoformed and a polylactic acid polymer sheet to be thermoformed which show good formability during thermoforming while maintaining impact resistance and heat resistance, and a thermoformed article obtained from the polylactic acid polymer sheet, according to the present invention, a polylactic acid polymer composition is used which comprises a mixture of an essentially amorphous polylactic acid polymer and a crystalline polylactic acid polymer, wherein the essentially amorphous polylactic acid polymer is present at the rate of more than 50% of the crystalline polylactic acid.

TECHNICAL FIELD

This invention relates to a biodegradable resin composition forthermoforming, polylactic acid polymer sheet for thermoforming, andthermoformed object obtained therefrom.

BACKGROUND ART

Blister packs, which are widely used to exhibit various kinds ofmerchandise, box-shaped packages formed by bending, shell type packagingcases, etc. are ordinarily formed by subjecting a resin sheet tothermoforming such as vacuum forming, air-pressure forming or hotbending. Packages for such foods as daily dishes, vegetables, sandwichesand lunch that are used e.g. in convenience stores are also formed bysimilar thermoforming.

For such blister packs, box-shaped packages, shell type packaging casesand food containers, transparent ones are especially preferred so thatthe contents can be seen therethrough. Thus, many blister packs areformed of sheets of polyvinyl chloride, polyethylene terephthalate orpolystyrene.

These materials have a problem in that because of their chemical andbiological stability, after discarded, they remain in the naturalenvironment practically without decomposing and accumulate or scatter inthe natural environment, thus polluting the biosphere. When they areburied, they shorten the life of the burial site because they don'tdecompose.

Now, to protect the environment, biodegradable materials are beingvigorously studied and developed. Among them, a polylactic acid resin isconsidered to be one of the most promising biodegradable materials.Since polylactic acid resins are biodegradable, they naturally hydrolyzein the earth or water and are decomposed by microorganisms into harmlesssubstances. Also, because polylactic acids are low in calorific value,they will not damage the furnace when burned. Since they are derivedfrom plants, they can help humans to become free from dependence on oil,whose underground reserves are expected to be depleted.

One problem with such polylactic acid resins is their low heatresistance. While sheets of polylactic acid resins and articles formedof such sheets are being transported in a truck or a ship, or stored ina storage house, especially in summer time, they tend to be deformedand/or fused to each other due to elevated temperature in the truck,ship or storage house.

Another problem with polylactic acid resins is their brittleness. Due totheir brittleness, it is difficult to use them in the form of sheets.

It is however known that by biaxially sheets for blister stretchingpolylactic acid sheets so that the sheets have orientation, and articlesformed from such sheets that have superior transparency, impactresistance and heat resistance are obtainable (patent publication 1).

In patent publication 1, there is a description that the DL ratio of thepolylactic acid may be selected from among the entire range, i.e. therange from 100/0 to 0/100 (paragraph [0020] of Patent document 1). Butin order for the polylactic acid sheet to have orientation, it isnecessary that the polylactic acid be crystallized, and it is well-knownto those of ordinary skill in the art that polylactic acid showscrystallinity only if it is made up virtually only of one of theL-lactic acid and D-lactic acid. From the fact that the D/L ratio of thepolylactic acid used in any of the examples of Patent document 1 is inthe range of 4-5/96-95, polylactic acid disclosed in Patent document 1are considered to be all crystallized.

It is also known that from a polylactic acid having a predeterminedsurface orientation (ΔP), heat of fusion (ΔHm), and heat of coldcrystallization (ΔHc), it is possible to form an article having superiorimpact resistance and moist heat resistance (Patent document 2). Sincethis polylactic acid has ΔHm and ΔHc, it is crystalline.

(Patent document 1: JP patent publication 8-73628 (see for example claim1 and paragraph [0077]); Patent document 2: JP patent publication9-25345 (see for example claim 1 and paragraph [0048])

DISCLOSURE OF THE INVENTION

In order for a sheet formed of a polylactic acid as disclosed in Patentdocument 1 or 2 to show sufficient impact resistance and heatresistance, it is often necessary that the sheet have a sufficientthickness. But when such a thick sheet is subjected to thermoforming,while it maintains good impact resistance and heat resistance, pressurenecessary during thermoforming tends to be high, so that formability maydeteriorate.

An object of the present invention is to provide a polylactic acidpolymer composition to be thermoformed which shows good formabilityduring thermoforming while maintaining sufficient impact resistance andheat resistance, a sheet formed of such a composition, and an articleobtained by thermoforming such a sheet.

According to this invention, in order to achieve this object, there isprovided a polylactic acid polymer composition to be subjected tothermoforming, the composition comprising a mixture of an essentiallyamorphous polylactic acid polymer and a crystalline polylactic acidpolymer, wherein the amorphous polylactic acid polymer is present in anamount of more than 50% of the crystalline polylactic acid polymer.

From another aspect of the invention, there is provided a polylacticacid polymer sheet to be subjected to thermoforming which is formed ofthe abovementioned polylactic acid polymer composition to be subjectedto thermoforming wherein the essentially amorphous polylactic acidpolymer comprises L-lactic acid and D-lactic acid of which the contentratio (L-lactic acid (%):D-lactic acid (%)) is 92:8 to 8:92, and thecrystalline polylactic acid polymer contains L-lactic acid and D-lacticacid of which the content ratio (L-lactic acid (%): D-lactic acid (%))is not less than 94:6 or not more than 6:94, the crystalline polylacticacid polymer being present in an amount of 10 to 200 parts by weightbased on 100 parts by weight of the amorphous polylactic acid polymer.

By using a mixture of an essentially amorphous polylactic acid polymerand a crystalline polylactic acid polymer, the composition exhibits highimpact resistance and heat resistance inherent to the crystallinepolylactic acid polymer. The amorphous polylactic acid polymer impartsflexibility to the composition, thereby improving formability duringthermoforming.

Preferably, in obtaining a thermoformed article by subjecting thepolylactic acid polymer sheet to thermoforming, the forming temperatureis determined so as to satisfy the following conditions (1) so that thethermoformed article has sufficient formability and shape retainability,impact resistance and heat resistance. Such an article can beadvantageously used as blister packs, food packaging containers, shelltype packaging cases, etc.

(1) Forming temperature that satisfies the relation 0.005<ΔHmf/ΔHm<0.5when the polylactic acid polymer sheet to be subjected to thermoformingis heated, where ΔHmf is a heat of fusion in a temperature range fromthe glass transition temperature to the forming temperature of thepolylactic acid polymer sheet to be subjected to thermoforming.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a DSC chart of the polylactic acid polymer sheet to besubjected to thermoforming according to the present invention; FIG. 2(a)is a plan view of a food container manufactured in Examples; FIG. 2(b)is a side view of FIG. 2(a); FIG. 2(c) is a plan view of a blister packmanufactured in Examples; FIG. 2(d) is a side view of FIG. 2(c); FIG.2(e) is a plan view of a shell type packaging case manufactured inExamples; and FIG. 2(f) is a side view of FIG. 2(e).

BEST MODE FOR EMBODYING THE INVENTION

The invention is now described more specifically. The polylactic acidpolymer composition to be subjected to thermoforming according to thepresent invention comprises a mixture of an essentially amorphouspolylactic acid polymer (hereinafter referred to as “resin A”) and acrystalline polylactic acid polymer (hereinafter referred to as “resinB”), with resin A present in an amount of more than 50 percent of resinB. The polylactic acid polymers forming resin A and resin B may behomopolymers of which the structural unit is L-lactic acid or D-lacticacid, i.e. poly(L-lactic acid) or poly(D-lactic acid), or copolymers ofwhich the structural unit comprises both L-lactic acid and D-lacticacid, i.e. poly(DL-lactic acid), or a mixture thereof.

The polylactic acid polymers may be polymerized by condensationpolymerization, ring-opening polymerization or any other known method.In condensation polymerization, a polylactic acid resin of a desiredcomposition is obtainable by directly subjecting L-lactic acid, D-lacticacid or a mixture thereof to dehydration/condensation polymerization.

In ring-opening polymerization, a polylactic acid polymer is obtainableby polymerizing lactides, which are cyclic dimers of lactic acids, inthe presence of a catalyst while optionally adding a polymerizationregulator. There are a plurality of lactides, i.e. L-lactide, which is adimer of L-lactic acid, D-lactide, which is a dimer of D-lactic acid,and DL-lactide, which comprises L-lactide and D-lactide. A polylacticacid polymer having a desired composition and crystallinity isobtainable by mixing and polymerizing necessary ones of lactides.

If e.g. higher heat resistance is required, small amounts of copolymercomponents may be added. It is also possible to use nonaliphaticdicarboxylic acids such as terephthalic acid, or nonaliphatic diols suchas ethylene oxide adducts of bisphenol A.

In order to increase the molecular weight, it is also possible to usesmall amounts of chain extenders such as diisocyanate compounds, epoxycompounds or acid anhydrides.

The polylactic acid polymers may also be copolymers of lactic acid andother hydroxy-carboxylic acid units such as α-hydroxy-carboxylic acid,or copolymers of lactic acid and aliphatic diols/aliphatic dicarboxylicacids. Such other hydroxy-carboxylic acid units include optical isomersof lactic acids (D-lactic acid for L-lactic acid, and L-lactic acid forD-lactic acid), glycolic acid, 3-hydroxybutyric acid, 4-hydroxybutyricacid, 2-hydroxy-n-butyric acid, 2-hydroxy-3,3-dimethylbutyric acid,2-hydroxy-3-methylbutyric acid, 2-methyllactic acid, bifunctionalaliphatic hydroxy-carboxylic acids such as 2-hydroxycaproic acid, andlactones such as caprolactone, butyrolactone and valerolactone.

Aliphatic diols which can be copolymerized into the polylactic acidpolymers include ethylene glycol, 1,4-butanediol, and1,4-cyclohexanedimethanol.

Aliphatic dicarboxylic acids include succinic acid, adipic acid, subericacid, sebacic acid and dodecanedioic acid.

Preferably, the polylactic acid polymers used in this invention have aweight-average molecular weight of 60000 to 700000, more preferably60000 to 400000, especially preferably 60000 to 300000. If theweight-average molecular weight is less than 60000, practical physicalproperties such as mechanical physical properties and heat resistancewill scarcely reveal themselves. If greater than 700000, the meltviscosity will be so high as to deteriorate formability.

The ratio (%) of the L-lactic acid and D-lactic acid in resin A, i.e.the essentially amorphous polylactic acid polymer is preferably from92:8 to 8:92, more preferably from 91:9 to 9:91. Outside of thispreferable range, the polylactic acid polymer tends to developcrystallinity, thereby making it difficult to thermoform the polylacticacid polymer sheet to be subjected to thermoforming that comprises thethus obtained polylactic acid polymer composition to be thermoformedinto desired shape.

The ratio (%) of the L-lactic acid and D-lactic acid in resin B, i.e.the crystalline polylactic acid polymer is preferably not less than 94:6or not more than 6:94, more preferably not less than 94.5:5.5 or notmore than 5.5:94.5. Outside of this range, the polylactic acid polymertends to be amorphous. Thus, it tends to be difficult to impartsufficient impact resistance and heat resistance to the polylactic acidpolymer composition obtained.

Resin B is preferably present in the mixture of resins A and B in anamount of 10 to 200 parts by weight, more preferably 20 to 100 parts byweight, based on 100 parts by weight of resin A. If less than 10 partsby weight, it tends to be difficult to impart sufficient impactresistance and heat resistance to the polylactic acid polymer obtained.If more than 200 parts by weight, it is sometimes difficult tothermoform the polylactic acid polymer sheet to be subjected tothermoforming that comprises the thus obtained polylactic acid polymercomposition to be thermoformed into desired shape.

To the polylactic acid polymer composition, besides resins A and B, analiphatic polyester other than a polylactic acid polymer may be added.Preferably, such an aliphatic polyester other than a polylactic acidpolymer is preferably present in an amount of 0.1 to 10 parts by weight,more preferably 0.5 to 7 parts by weight, based on 100 parts by weightof the total amount of resins A and B. Within this range, it is possibleto further improve the impact resistance of the composition withoutsignificantly deteriorating its transparency and formability. If such analiphatic polyester other than a polylactic acid polymer is present inan amount of less than 0.1 parts by weight, the impact resistance willnot improve sufficiently. If it is present in an amount of more than 10parts by weight, transparency tends to be insufficient if transparencyis required.

Preferably, such an aliphatic polyester other than a polylactic acidpolymer has a glass transition temperature (Tg) not exceeding 0 degreesCelsius and a melting point (Tm) of not less than 60 degrees Celsius.

If the Tg value is higher than 0 degrees Celsius, the impact resistanceof the sheet obtained from the polylactic acid polymer composition to bethermoformed will not improve sufficiently. Also, its transparency willdeteriorate because the aliphatic polyester forms spherulites. If the Tmvalue is lower than 60 degrees Celsius, the heat resistance will beinsufficient.

The aliphatic polyester other than a polylactic acid polymer may be analiphatic polyester obtained by condensing an aliphatic diol and analiphatic dicarboxylic acid, an aliphatic polyester obtained bysubjecting a cyclic lactone to ring-opening polymerization, a syntheticaliphatic polyester or an aliphatic polyester synthesized in microbes.

The aliphatic polyester obtained by condensing an aliphatic diol and analiphatic dicarboxylic acid is obtained by condensing, as an aliphaticdiol or diols, one or more of ethylene glycol, 1,4-butanediol,1,4-cyclohexanedimethanol, etc. and as an aliphatic dicarboxylic acid oracids, one or more of succinic acid, adipic acid, suberic acid, sebacicacid, dodecanedioic acid, etc. A desired polymer is then obtained byoptionally extending chains with e.g. isocyanate compounds.

The aliphatic polyester obtained by subjecting cyclic lactones toring-opening polymerization is typically obtained by polymerizing, as acyclic monomer or monomers, one or more of ε-caprolactone,δ-valerolactone, β-methyl-δ-valerolactone, etc.

Synthetic aliphatic polyesters include copolymers of cyclic anhydridesand oxiranes, such as copolymers of succinic anhydride and ethyleneoxide or propylene oxide.

Among known aliphatic polyesters synthesized in microbes is an aliphaticpolyester synthesized in microbes, such as Alcaligenes eutrophus by theaction of acetyl coenzyme A. While this aliphatic polyester mainlycomprises poly-β-hydroxybutyric acid (poly-3HB), it is industriallyadvantageous to copolymerize it with valeric acid units (HV) into acopolymer of poly(3HB-co-3HV), thereby improving its practicality as aplastic. The HV copolymerization ratio is ordinarily 0 to 40 percent. Ahydroxyalkanoate having longer chains may be copolymerized, too.

Specific aliphatic polyesters other than polylactic acid polymers may beat least one substance selected from the group consisting ofpolybutylene succinate, polybutylene succinate adipate, polybutyleneadipate terephthalate, polycaprolactone, polyglycolic acid, polyestercarbonate, copolymers of polyhydroxybutyrate and polyhydroxyvalerate,and copolymers of polyhydroxybutyrate and polyhydroxyhexanoate.

The polylactic acid polymer composition according to the presentinvention may be produced by blending or extruding/pelletizing.

If it is produced by blending, resins A and B, which form the polylacticacid polymer composition, and optionally, the aliphatic polyester otherthan a polylactic acid polymer as a impact resistance modifier, andinorganic particles are sufficiently dried and blended in a blender toform pellets as the composition according to the invention.

If the composition is produced by extruding/pelletizing, resins A and B,which form the polylactic acid polymer composition to be thermoformed,and optionally, the aliphatic polyester other than a polylactic acidpolymer as a impact resistance modifier, and inorganic particles aresufficiently dried and supplied to an extruder. The melt extrusiontemperature should be determined taking into consideration the fact thatthe melting point of the polylactic acid polymer composition to bethermoformed varies with the ratio of the L-lactic acid and D-lacticacid, and the melting point and the mixture ratio of the aliphaticpolyester other than a polylactic acid polymer. Actually, the meltextrusion temperature is preferably 100 to 250 degrees Celsius. Then,the mixture is extruded in the form of strands, and cut into pellets bya strand cutter. A desired polylactic acid polymer composition to bethermoformed is thus obtained.

Of the above two methods, the latter is preferable to uniformly mix thecomponents, though care must be taken not to reduce the molecular weightdue to decomposition.

In order to improve various physical properties, to the polylactic acidpolymer composition to be thermoformed, heat stabilizers, lightstabilizers, light absorbers, plasticizers, inorganic fillers,colorants, pigments, etc may be added.

A polylactic acid polymer sheet to be thermoformed may be produced fromthe polylactic acid polymer composition by any ordinary method such asextrusion or casting. But for higher production efficiency, extrusion ispreferable. Optionally, the polylactic acid polymer composition may beused as an outer layer or an intermediate layer of a laminate of whichthe other layers are formed of other biodegradable resin compositions sothat the laminate has printability or heat sealability.

The sum of the thicknesses of the layers formed of other biodegradableresin compositions should be within such a range as not to deteriorateformability of the laminate obtained. Specifically, it is preferably notmore than 30%, more preferably not more than 20%, of the thickness ofthe laminate. If this value is greater than 30%, the thermoformabilityof the laminate may deteriorate.

Description is now made on specifically how the polylactic acid polymersheet to be thermoformed is produced from the polylactic acid polymercomposition to be thermoformed by extrusion.

The polylactic acid polymer composition to be thermoformed is driedsufficiently to remove moisture, melted in an extruder and extruded froma die. The melt extrusion temperature should be determined taking intoconsideration the fact that the melting point of the polylactic acidpolymer composition to be thermoformed varies with the ratio of theL-lactic acid and D-lactic acid, and the melting point and the mixtureratio of the aliphatic polyester. Specifically, the melt extrusiontemperature is preferably 100 to 250 degrees Celsius.

If the composition is laminated on layers of other biodegradable resincompositions in order to improve printability and heat sealability, itmay be laminated using two or more multimanifolds or feed blocks andextruded from a slit-shaped die in the form of a two or more layers ofmolten sheet. The thicknesses of the respective layers can be controlledby adjusting the flow rate of polymers with fixed-quantity feeders suchas gear pumps.

Then, the single-layer or multi-layer molten sheet extruded from the dieis rapidly cooled to a temperature below its glass transitiontemperature on a rotary cooling drum to obtain an essentially amorphous,non-oriented polylactic acid polymer sheet to be thermoformed. In orderto improve smoothness and uniformity in thickness of the polylactic acidpolymer sheet to be thermoformed, the polymer sheet is preferablybrought into close contact with the rotary cooling drum as closely aspossible by e.g. an electrostatic application method or a liquid coatingmethod.

The polylactic acid polymer sheet thus obtained has no sufficient impactresistance and heat resistance yet. It is possible to increase theimpact resistance and heat resistance to a sufficient level bycrystallizing resin B in the polylactic acid polymer sheet to bethermoformed when the sheet is thermoformed into a thermoformed article.But it is also possible to increase the impact resistance and heatresistance before thermoforming the sheet by e.g. heating and thengradually cooling the sheet, thereby crystallizing resin B in thepolylactic acid polymer sheet, or uniaxially or biaxially stretching thesheet to crystallize resin B in the sheet. In this case, the sheetmaintains high impact resistance and heat resistance even after it hasbeen thermoformed into a thermoformed article.

Surface orientation (ΔP) can be used as an index of orientation of thebiaxially oriented polylactic acid polymer sheet to be thermoformed. Thevalue ΔP is preferably 3.0×10⁻³ to 30×10⁻³, more preferably 3.0×10⁻³ to15×10⁻³. If this value is less than 3.0×10⁻³, the heat resistance andimpact resistance tend to be insufficient. If greater than 30×10⁻³,continuity of orientation may be lost, or the orientation tends to be solarge as to deteriorate thermoformability.

In order for the value ΔP to be within the above preferred range, thesheet is preferably stretched at least in one direction at a temperatureof 50 to 100 degrees Celsius by 1.5 to 5 times at a rate of 100 to 10000%/minute.

The thus stretched polylactic acid polymer sheet to be thermoformed issubjected to heat treatment while being fixed to obtain a heat-fixed,stretched/oriented sheet. By heat-fixing the sheet, sagging of the sheetis prevented. The heat resistance of the sheet improves, too.

The heat-fixing temperature and time are determined such that thedifference between the heat of fusion ΔHm and the heat of coldcrystallization ΔHc, i.e. (ΔHm·ΔHc) is preferably 5 to 20 J/g, morepreferably 7 to 20 J/g, and the value {(ΔHm·ΔHc)/ΔHm} is preferably notless than 0.85, more preferably not less than 0.90.

If the value (ΔHm·ΔHc) is less than 5 J/g, it is difficult for thepolylactic acid polymer sheet to have sufficient heat resistance. Ifgreater than 20 J/g, no sufficient thermoformability may be achievable.

If the value {(ΔHm·ΔHc)/ΔHm} of the polylactic acid polymer sheet isless than 0.85, it is difficult for the sheet to have sufficient heatresistance. The theoretical upper limit of this value is 1.0.

The heat-fixing temperature is preferably not less than the glasstransition temperature (Tg) of the polylactic acid polymer compositionand not more than its melting temperature (Tm), more preferably not lessthan Tg+20 degrees Celsius and not more than Tm minus 10 degreesCelsius, further preferably not less than Tg+40 degrees Celsius and notmore than Tm minus 20 degrees Celsius.

If the heat-fixing temperature is lower than Tg, the sheet will notsufficiently crystallize, so that its heat resistance tends to be poor.If this temperature is higher than Tm, the sheet may break during heatfixing.

The shrinkage of the biaxially stretched polylactic acid polymer sheetmanufactured in the above-described manner is preferably not more than10 percent, more preferably not more than 5 percent, both in itslongitudinal and transverse directions, when left in an environment of90 degrees Celsius for 30 minutes. If the shrinkage is over 10 percent,the sheet tends to suffer from heat shrinkage during thermoforming.

The thickness of the polylactic acid polymer sheet obtained in the abovemanner is not particularly limited provided the sheet can be used inordinary thermoforming. Specifically, its thickness is preferably withinthe range of 0.03 to 2.0 mm. Out of this range, thermoforming may besometimes difficult.

The polylactic acid polymer sheet obtained in this manner has suitablethermoformability, and can be subjected to thermoforming with a generalpurpose forming machine. By preheating the sheet to a thermoformingtemperature with e.g. an infrared heater, a hot plate heater, or by hotair, and thermoforming it, the sheet can be formed into blister packs,containers, etc.

Specific thermoforming methods include vacuum forming, plug-assistedforming, air-pressure forming, a method using male and female dies, anda method in which the sheet is deformed along a male die and then themale die is expanded. The sheet can be formed into any desired shape andsize according to the intended use of the final product.

The thermoforming temperature of the polylactic acid polymer sheet ispreferably within a range that satisfies the requirement (1) below.

(1) Forming temperature that satisfies the relation 0.005<ΔHmf/ΔHm<0.5when said polylactic acid polymer sheet to be subjected to thermoformingis heated, where ΔHmf is a heat of fusion in a temperature range fromthe glass transition temperature to said forming temperature of saidpolylactic acid polymer sheet to be subjected to thermoforming.

The value ΔHmf is the heat of fusion at a portion of a peak thatindicates ΔHm in the chart of FIG. 1, which is obtained by differentialscanning calorimetry when the polylactic acid polymer sheet is heated.More specifically, the value ΔHmf is the heat of fusion at a portion ofthe abovementioned peak within a temperature range from the glasstransition temperature to the forming temperature (hatched portion inFIG. 1).

As described above, the value ΔHmf/ΔHm is preferably in the range of0.005 to 0.5, more preferably 0.01 to 0.3. Within this range, the sheetcan be easily thermoformed into a desired shape, and the articleobtained by thermoforming is high in heat resistance and impactresistance. If the value ΔHmf/ΔHm is less than 0.005, the sheet tends tosuffer from massive strains during thermoforming, thus deteriorating theheat resistance of the thermoformed article. If the value ΔHmf/ΔHm isgreater than 0.5, more than half of the oriented crystallized portion inthe sheet begins to melt. Thus, while formability improves, theorientation of the sheet tends to be lost, thus deteriorating the impactresistance.

Because the value ΔHm is the heat of fusion necessary to melt orientedcrystals derived from resin B, when the sheet is subjected tothermoforming such that the value ΔHmf satisfies the above conditions,resin A is presumably present in the matrix of resin B, which maintainscrystals in a suitable amount, in a substantially molten state.

Thus, if a single crystalline polylactic acid polymer having the sameD-lactic acid content as the average D-lactic acid content of thepolylactic acid polymer sheet is used, the melting point itself mayfall, resulting in insufficient thermoformability.

Description is now made on how the polylactic acid polymer sheet issubjected to air-pressure forming as thermoforming.

With both ends thereof chucked by chains with jaws, the polylactic acidpolymer sheet is introduced into a far-infrared heater furnace to heatthe sheet, and with the sheet kept chucked, the sheet is introduced intobetween upper and lower dies. Then, the dies are closed to press thesheet. The article formed by pressing the sheet is taken out of thedies, and the chucks are released. Since the polylactic acid polymersheet is subjected to thermoforming while applying tension thereto usingthe chucks, not only shrinkage in the width direction of the polylacticacid polymer sheet but also shrinkage in the flow direction of the sheetcan be suppressed to a level at which there will be no substantialinfluence on the sheet.

Since the polylactic acid polymer sheet comprises a layer comprising apolylactic acid polymer, it is safe to humans, and can be used ascontainers to be brought into direct contact with food.

Any excess material produced in any of the abovementioned steps can berecycled as a portion of the polylactic acid polymer composition.

The thermoformed article according to the present invention can be usedas blister packs, which are widely used to exhibit various kinds ofmerchandise, box-shaped packages formed by bending, shell type packagingcases, etc. It can also be used as packages for such foods as dailydishes, vegetables, sandwiches and lunch that are used e.g. inconvenience stores.

EXAMPLES

Examples of the invention and comparative examples are now described. Itis to be understood that the present invention is not limited to theseexamples. Examples were evaluated for the following items.

[Evaluated Items]

(1) Weight-average Molecular Weight

Using a gel permeation chromatograph HLC-8120GPC made by TosohCorporation, a calibration curve of standard polystyrene was createdunder the following conditions to measure the weight-average molecularweight.

Column used: Shim-Pack series made by Shimadzu Corporation (GPC-801C,

GPC-804C, GPC-806C, GPC-8025C and GPC-800CP)

Solvent: chloroform

Concentration of sample solution: 0.2 wt/vol %

Amount of sample solution injected: 200 microliters

Flow rate of solvent: 1.0 ml/minute

Temperature of the pump, column and detector: 40 degrees Celsius

(2) Thickness of the Sheet

As the thickness of the sheet, the average of thicknesses measured atten points of the sheet using a dial gauge SM-1201 made by Teclock wascalculated in micrometers.

(3) Heat Resistance

The article obtained by thermoforming the sheet was left in a hot-airoven which was kept at 60 degrees Celsius for 30 minutes, and the volumereduction rate (%) was calculated.

Volume reduction rate (%)={1−(volume of the formed article after heattreatment/volume of the formed article before heat treatment)}×100

The symbols used in Table 2 have the following meanings.

◯: The volume reduction rate was less than 3%.

Δ: The volume reduction rate was not more than 6% and not less than 3%.

x: The volume reduction rate was over 6%.

(4) Impact Resistance of the Sheet

Using a hydroshot impact tester (model HTM-1) made by Toyo SeikiSeisaku-Sho, Ltd., a member similar in shape to a firing pin of firearmsand having a diameter of a half inch were hit against the sheet at aspeed of 3 meters/second at 23 degrees C. to calculate the energynecessary to destroy the sheet. The symbols in Table 2 have thefollowing meanings:

◯: The impact value necessary to break the sheet was not less than 20kg·mm.

Δ: The impact value necessary to break the sheet was not less than 10kg·mm and less than 20 kg·mm.

x: The impact value necessary to break the sheet was less than 10 kg·mm.

(5) Impact Resistance of the Formed Article

The article obtained by thermoforming the sheet was filled with water,and after sealing its opening, it was dropped onto a concrete floor fromthe height of 1 meter. The article was then checked for any damage.

(6) Determination of the Glass Transition Temperature (Tg) and theMelting Point (Tm) of the Sheet

Under JIS-K-7121, the glass transition temperature (Tg) and the meltingpoint (Tm) of the sheet were measured by differential scanningcalorimetry (DSC) while heating the sheet at a heating rate of 10degrees Celsius/minute.

(7) Determination of Crystallinity

Under JIS-K-7121, the heat of fusion (ΔHm) and the heat of coldcrystallization (ΔHc) that both originate from the polylactic acid resinin the sheet were measured while heating the sheet at a heating rate of10 degrees Celsius/minute. From these values, the crystallinity of thepolylactic acid polymer was calculated as follows:

Relative crystallinity (%)=(ΔHm·ΔHc)/ΔHm×100

The value ΔHm was measured after the sheet has been subjected to heattreatment at 120 degrees Celsius for four hours, too.

(8) Calculation of ΔHmf (Forming Temperature)

Based on the temperature-dependent endothermic and exothermic chartobtained by differential scanning calorimetry (DSC), the formingtemperature was calculated by converting the value ΔHm and the valueΔHmf to the predetermined forming temperature (i.e. calculated from thevalues ΔHm and ΔHmf/ΔHm).

(9) Measurement of Haze

Haze was measured under JIS K 7105. The symbols in Table 2 have thefollowing meanings:

◯: The haze was not more than 10%.

Δ: The haze was over 10% and not more than 20%.

x: The haze was over 20%.

(10) Formability

Air-pressure forming (air pressure: 4 kg/cm²) was carried out using amold measuring 100 mm in diameter and 30 mm deep and having a drawingratio of 0.3 (mold temperature: 60 degrees Celsius). The shape of thearticle thus formed was observed, and evaluated in three stages.

◯: Good

Δ: Practically passable

x: Not good

For heating, an INFRASTEIN heater was used. The temperature of eachsheet immediately before forming was measured using a far-infraredthermometer.

(11) Summary Evaluation

The sheet of each example was formed into a food container shown inFIGS. 2(a) and 2(b), a blister pack shown in FIGS. 2(c) and 2(d), and ashell-type packaging case shown in FIGS. 2(e) and 2(f).

The symbols in Table 2 have the following meanings:

◯: Practically usable

Δ: Barely practically passable

x: Not practically usable

[Production of Polylactic Acid Polymer]

Production Example 1

100 kg of L-lactide made by Purac Japan (trade name: PURASORB L) with 15ppm of tin octylate added was put in a 500-liter batch typepolymerization tank having an agitator and a heater. In thepolymerization tank, after nitrogen substitution, the L-lactide waspolymerized at 185 degrees C. for 60 minutes while being agitated at aspeed of 100 rpm. The thus obtained molten product was supplied to aparallel twin screw extruder having a diameter of 40 mm and includingvacuum vents in three stages (made by Mitsubishi Heavy Industries,Ltd.), and was extruded in the form of strands at 200 degrees C. whilebeing deaerated at a vent pressure of 4 Torr. The strands were thenpelletized.

The thus obtained polylactic acid polymer (hereinafter referred to as“B1”) had a weight-average molecular weight of 200000, and its L-lacticacid content was 99.5%. The melting point measured by DSC was 171degrees C.

Production Examples 2 and 3

94 kg of L-lactide (trade name: PURASORB L) and 6 kg of DL-lactide(trade name: PURASORB DL), both made by Purac Japan, with 15 ppm of tinoctylate added were put in a 500-liter batch type polymerization tankhaving an agitator and a heater. In the polymerization tank, afternitrogen substitution, the mixture was polymerized at 185 degrees C. for60 minutes while being agitated at a speed of 100 rpm. The thus obtainedmolten product was supplied to a parallel twin screw extruder having adiameter of 40 mm and including vacuum vents in three stages (made byMitsubishi Heavy Industries, Ltd.), and was extruded in the form ofstrands at 200 degrees C. while being deaerated at a vent pressure of 4Torr. The strands were then pelletized.

The thus obtained polylactic acid polymer (hereinafter referred to as“B2”) had a weight-average molecular weight of 200000, and its L-lacticacid content was 97.0%. The melting point measured by DSC was 168degrees C.

A polylactic acid polymer (hereinafter “B3”) having a weight-averagemolecular weight of 200000 and an L-lactic acid content of 94.8% wasprepared in the same manner as B2 except that the amounts of theL-lactide and DL-lactide were changed. Its melting point, as measured byDSC, was 165 degrees C.

Production Examples 4 and 5

80 kg of L-lactide (trade name: PURASORB L) and 20 kg of DL-lactide(trade name: PURASORB DL), both made by Purac Japan, with 15 ppm of tinoctylate added were put in a 500-liter batch type polymerization tankhaving an agitator and a heater. In the polymerization tank, afternitrogen substitution, the mixture was polymerized at 185 degrees C. for60 minutes while being agitated at a speed of 100 rpm. The thus obtainedmolten product was supplied to a parallel twin screw extruder having adiameter of 40 mm and including vacuum vents in three stages (made byMitsubishi Heavy Industries, Ltd.), and was extruded in the form ofstrands at 200 degrees C. while being deaerated at a vent pressure of 4Torr. The strands were then pelletized.

The thus obtained polylactic acid polymer (hereinafter referred to as“A1”) had a weight-average molecular weight of 200000, and its L-lacticacid content was 89.7%.

A polylactic acid polymer (hereinafter “A2”) having a weight-averagemolecular weight of 200000 and an L-lactic acid content of 79.6% wasprepared in the same manner as A1 except that the amounts of theL-lactide and DL-lactide were changed. Neither A1 nor A2 had a crystalmelting point, as measured by DSC, which means that they were bothamorphous.

Production Example 6

85 kg of L-lactide (trade name: PURASORB L) and 15 kg of DL-lactide(trade name: PURASORB DL), both made by Purac Japan, with 15 ppm of tinoctylate added were put in a 500-liter batch type polymerization tankhaving an agitator and a heater. In the polymerization tank, afternitrogen substitution, the mixture was polymerized at 185 degrees C. for60 minutes while being agitated at a speed of 100 rpm. The thus obtainedmolten product was supplied to a parallel twin screw extruder having adiameter of 40 mm and including vacuum vents in three stages (made byMitsubishi Heavy Industries, Ltd.), and was extruded in the form ofstrands at 200 degrees C. while being deaerated at a vent pressure of 4Torr. The strands were then pelletized.

The thus obtained polylactic acid polymer (hereinafter referred to as“C”) had a weight-average molecular weight of 200000, and its L-lacticacid content was 92.6%. The crystalline melting point, as measured byDSC, was 131 degrees C.

Data on the polylactic acid polymers produced in the respectiveProduction Examples are shown in Table 1. TABLE 1 Rate of Rate ofD-lactic L-lactic Weight-average Glass Melting ΔHm Polylactic acid acidmolecular transition point (*1) acid polymer (%) (%) weight temperature(° C.) Crystallinity (J/g) Resin A A1 10.3 89.7 200,000 53 — Amorphous 0A2 20.4 79.6 200,000 52 — Amorphous 0 Resin B B1 0.5 99.5 200,000 56 178High 50 crystalline B2 2.0 98.0 200,000 56 162 High 42 crystalline B35.2 94.8 200,000 56 145 Medium 35 crystalline Other C 7.4 92.6 200,00053 131 Low 13 crystalline(*1) Here, ΔHm is the value after the pellets obtained in eachproduction example has been subjected to heat treatment at 120 degreesC. for four hours.

Examples of the Invention 1-5 and Comparative Examples 1-4

Resins A and B shown in Table 2 were sufficiently dried, and mixedtogether at the rates shown in Table 2. Dried particulate silica havingan average particle diameter of 1.4 micrometers (trade name: Silysia100, made by Fuji Silysia Chemical, Ltd.) was added by 0.1 parts byweight based on 100 parts by weight of the total amount of resins A andB, and the mixture was extruded in the form of strands by a paralleltwin screw extruder having a diameter of 25 mm. The strands were thenwater-cooled and cut into 2-milimeter-long pellets of a polylactic acidpolymer composition by a pelletizer. The thus obtained pellets weresubjected to aging at 120 degrees C. for 4 hours. Then, the heat offusion ΔHm necessary to melt crystals was measured for each example byDSC. The results are shown in Table 2.

The pellets obtained were sufficiently dried, and extruded at 210degrees C. through a coat hanger type mouthpiece with a parallelextruder having a diameter of 75 mm in the shape of a sheet. The thuscoextruded sheet was cooled rapidly with a casting roll kept at about 43degrees C. to obtain an unstretched sheet. Using a film tenter made byMitsubishi Heavy Industries, Ltd., this sheet was heated to 75 degreesC. with an infrared heater while kept in contact with a hot watercirculation roll and stretched longitudinally by 2.0 times between rollsthat are rotating at different peripheral speeds. The sheet was thenguided into the tenter while being held by clips and stretched in thedirection transverse to the feed direction of the sheet by 2.8 times at75 degrees C. The thus stretched sheet was subjected to heat treatmentat 135 degrees C. for 30 seconds while applying tension thereto. A sheethaving a thickness of 300 micrometers was obtained. The shrinkage rateof the sheet thus obtained was not more than 5 percent both in thelongitudinal and transverse directions when placed in a 90 degrees C.oven for 30 minutes. The sheet thus showed good high-temperaturedimensional stability. The values ΔHm, ΔHc, Tg and Tm of the sheetobtained were measured by DSC.

The sheet obtained were preheated to 145 degrees C., and subjected toair-pressure forming (air pressure: 4 kg/cm²) in a mold (moldtemperature: 60 degrees C.) measuring 100 mm in diameter and 30 mm deepand having a drawing ratio of 0.3.

Various parameters were measured and assessed during the abovementionedvarious steps. They are shown in Table 2. TABLE 2 Examples of theinvention Comparative Examples 1 2 3 4 5 1 2 3 4 Polylactic acid polymerResin A A1 A1 A1 A1 A2 A1 A1 A1 A1 (parts by 100 100 100 100 100 100 1000 0 weight) D % 10.3 10.3 10.3 10.3 20.4 10.3 10.3 10.3 10.3 Resin B B1B1 B1 B3 B3 B1 B1 B3 (parts by 100 50 30 20 200 200 10 100 weight) D %0.5 0.5 0.5 5.2 5.2 0.5 0.5 5.2 Other resins C (parts by 100 weight) D %7.4 Composition Average D % 5.4 7.0 8.0 9.5 10.3 3.8 9.4 5.2 7.4 Tg (°C.) 56 56 55 53 53 56 53 56 53 Tm (° C.) 177 176 176 144 145 178 176 145131 ΔHm (*1) (J/g) 25 17 12 6 23 33 5 35 13 Sheet Stretching heat (° C.)135 135 120 120 135 135 90 135 120 fixing temperature Thickness (mm) 0.30.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 Haze ◯ ◯ ◯ ◯ ◯ ◯ ◯ ◯ ◯ Impact resistance◯ ◯ ◯ Δ Δ ◯ X ◯ Δ Tg (° C.) 56 56 55 53 53 56 53 56 53 Tm (° C.) 177 177177 145 145 178 177 145 131 ΔHm (J/g) 23 15 11 5 22 11 4 45 13 ΔHc (J/g)2.5 1.0 0.5 0.2 1.0 2.0 0.8 2.0 1.4 ΔHm − ΔHc 20 14 10 5 20 29 3 43 12(ΔHm − ΔHc)/ΔHm 0.9 0.95 0.95 0.95 0.95 0.95 0.8 0.95 0.9 Sheettemperature (° C.) 145 145 145 120 120 145 120 130 110 during formingΔHmf/ΔHm 0.2 0.2 0.2 0.1 0.3 0.2 0.1 0.4 0.3 Heat-formed article Airpressure applied (kg/cm²) 4 4 4 4 4 4 4 4 4 Formability Δ Δ ◯ ◯ Δ X X X◯ Transparency ◯ ◯ ◯ ◯ ◯ — ◯ — ◯ Impact resistance ◯ ◯ ◯ Δ ◯ — X — ΔHeat resistance ◯ ◯ Δ Δ ◯ — X — X General Blister packs ◯ ◯ ◯ Δ Δ X X XX assessment Food containers Δ ◯ ◯ Δ Δ X X X X Shell type packaging Δ ◯◯ Δ Δ X X X X cases(*1) Here, ΔHm is a value after heat treatment for four hours at 120degrees C.

INDUSTRIAL APPLICATION

Because the polylactic acid polymer composition to be thermoformedaccording to this invention comprises a mixture of an essentiallyamorphous polylactic acid polymer and a crystalline polylactic acidpolymer, it exhibits impact resistance and heat resistance inherent tothe crystalline polylactic acid polymer. At the same time, thecomposition has flexibility attributable to the amorphous polylacticacid polymer.

The sheet to be thermoformed obtained from the polylactic acid polymercomposition according to the invention, and the article obtained bythermoforming this sheet are superior in impact resistance, heatresistance and transparency.

The thermoformed article according to the present invention can be usedas various end products such as blister packs, food containers and shelltype packaging cases.

1. A polylactic acid polymer composition to be subjected tothermoforming, said composition comprising a mixture of an essentiallyamorphous polylactic acid polymer and a crystalline polylactic acidpolymer, wherein said amorphous polylactic acid polymer is present in anamount of more than 50% of said crystalline polylactic acid polymer. 2.A polylactic acid polymer sheet to be subjected to thermoforming whichis formed of the polylactic acid polymer composition to be subjected tothermoforming of claim 1 wherein said essentially amorphous polylacticacid polymer comprises L-lactic acid and D-lactic acid of which thecontent ratio (L-lactic acid (%): D-lactic acid (%)) is 92:8 to 8:92,and said crystalline polylactic acid polymer contains L-lactic acid andD-lactic acid of which the content ratio (L-lactic acid (%) : D-lacticacid (%)) is not less than 94:6 or not more than 6:94, said crystallinepolylactic acid polymer being present in an amount of 10 to 200 parts byweight based on 100 parts by weight of said amorphous polylactic acidpolymer.
 3. The polylactic acid polymer sheet to be subjected tothermoforming of claim 2 which has a heat of fusion ΔHm when the sheetis heated and a heat of cold crystallization ΔHc produced due tocrystallization during heating, wherein the value (ΔHm−ΔHc) is 5 to 20J/g, and the value {(ΔHm−ΔHc)/ΔHm } is not less than 0.85.
 4. Thepolylactic acid polymer sheet to be subjected to thermoforming of claim2 wherein said polylactic acid polymer composition to be subjected tothermoforming contains an aliphatic polyester other than a polylacticacid polymer in an amount of 0.1 to 10 parts by weight per 100 parts byweight of the total amount of said essentially amorphous polylactic acidpolymer and said crystalline polylactic acid polymer.
 5. An articleobtained by subjecting the polylactic acid polymer sheet of claim 2 tothermoforming at a forming temperature that satisfies the belowconditions (1). (1) Forming temperature that satisfies the relation0.005<ΔHmf/ΔHm<0.5 when said polylactic acid polymer sheet to besubjected to thermoforming is heated, where ΔHmf is a heat of fusion ina temperature range from the glass transition temperature to saidforming temperature of said polylactic acid polymer sheet to besubjected to thermoforming.
 6. The polylactic acid polymer sheet to besubjected to thermoforming of claim 3 wherein said polylactic acidpolymer composition to be subjected to thermoforming contains analiphatic polyester other than a polylactic acid polymer in an amount of0.1 to 10 parts by weight per 100 parts by weight of the total amount ofsaid essentially amorphous polylactic acid polymer and said crystallinepolylactic acid polymer.
 7. An article obtained by subjecting thepolylactic acid polymer sheet of claim 3 to thermoforming at a formingtemperature that satisfies the below conditions (1). (1) Formingtemperature that satisfies the relation 0.005<ΔHmf/ΔHm<0.5 when saidpolylactic acid polymer sheet to be subjected to thermoforming isheated, where ΔHmf is a heat of fusion in a temperature range from theglass transition temperature to said forming temperature of saidpolylactic acid polymer sheet to be subjected to thermoforming.
 8. Anarticle obtained by subjecting the polylactic acid polymer sheet ofclaim 4 to thermoforming at a forming temperature that satisfies thebelow conditions (1). (1) Forming temperature that satisfies therelation 0.005<ΔHmf/ΔHm<0.5 when said polylactic acid polymer sheet tobe subjected to thermoforming is heated, where ΔHmf is a heat of fusionin a temperature range from the glass transition temperature to saidforming temperature of said polylactic acid polymer sheet to besubjected to thermoforming.
 9. An article obtained by subjecting thepolylactic acid polymer sheet of claim 6 to thermoforming at a formingtemperature that satisfies the below conditions (1). (1) Formingtemperature that satisfies the relation 0.005<ΔHmf/ΔHm<0.5 when saidpolylactic acid polymer sheet to be subjected to thermoforming isheated, where ΔHmf is a heat of fusion in a temperature range from theglass transition temperature to said forming temperature of saidpolylactic acid polymer sheet to be subjected to thermoforming.